Hi Meena,
Sometimes the hemoglobin drop is a result of illness, like colds or infections, but it will still require transfusions. Although they have said every three weeks, I am sure that can be modified if the post-transfusion drop in Hb is slow. If he maintains his Hb level after a transfusion, the next transfusion will not be so soon. But even if it is, there is no reason to worry. Chelation, when it starts, can remove whatever iron is built up by that point and in the meantime, I would highly recommend that you pay attention to what we advise about supplementing with antioxidants, as these can help prevent the damage that iron causes. This can make a real difference as you wait until the age when you can start chelation.
There is no reason to believe that he will need to transfuse anymore frequently than what you have been told. As I said, the Hb drop may be related to other factors and once he does begin regular transfusion, there is no reason his frequency would be more often, as long as antibody issues are avoided. Talk to your doctor about genotype matching of blood in addition to phenotype matching, as this can do much to prevent antibody reactions. This is especially important as the bulk of donor blood will be from different genetic backgrounds than your son, so the risk of antibody reaction is higher without genotype matching.